1. Field of the Invention
The present invention relates to a balance control circuit for cancelling the unbalance between the aural signals from a plurality of channels.
2. Description of the Related Art
In a stereo system for transmitting the right and left stereo signals by using different channels, an unbalance is sometimes caused due to the nonuniformity in the circuit or element which constitutes each circuit. For example, it is necessary that the voice of an announcer is output at the same intensity from the right and left speakers and constantly assigned to the center of both speakers. However, if an unbalance is caused between the aural signals from the channels as described above, the signals are assigned to a position which deviates from the center to the right or left, so that the voice output is unpleasant to the ear.
To deal with this problem, a balance control circuit for balancing the aural signals from the right and left channels have conventionally been proposed and put to practical use. FIG. 1 shows such a conventional balance control unit. A left stereo signal input to a left input terminal 1 is output from a left output terminal 3 through a left attenuator 2. A right stereo signal input to a right input terminal 4 is output from a right output terminal 6 through a right attenuator 5. The levels of the left and right stereo signals at the left and right output terminals 3, 6 are detected and compared with each other by a detector 7. An output signal which corresponds to the difference in the levels of the left and right stereo signals is output from the output terminal of the detector 7. The output signal is held by a holding circuit 8 and supplied to a controller 9. The controller 9 receives the output signal and controls the amounts of attenuation of the left and right attenuators 2, 5. When left and right stereo signals which correspond to the voice of an announcer are input to the left and right input terminals 1, 4, left and light stereo signals having the same level must be output from the left and right output terminals 3, 6. If there is unbalance between the signals from both channels, however, the levels of the left and right stereo signals are not equal. For example, if the level of the left stereo signal is higher than that of the right stereo signal, an output signal having a level higher than a predetermined level is output from the detector 7 and supplied to the controller 9 through the holding circuit 8. The controller 9 then generates a control signal and controls the balance by increasing the amount of attenuation of the left attenuator 2. On the other hand, if the level of the left stereo signal is lower than that of the right stereo signal, the controller 9 supplies an output signal for increasing the amount of attenuation of the right attenuator 5. In this way, the signal levels on the left and right channels are balanced.
Each element of the balance control circuit shown in FIG. 1 is composed of an analog circuit, and the holding circuit 8 for holding the output signal of the detector 7 is essential. However, since the holding circuit 8 is composed of a capacitor 10 and resistors 11, 12, as shown in FIG. 1, it is impossible to hold the output signal of the detector 7 for a long time. In addition, when the level of the output signal of the detector 7 rapidly changes, since the capacitor 10 is rapidly charged or discharged, shock noise is disadvantageously produced.
As a player constituting a signal source for the stereo system, various players such as a compact disk player and a video disk player are used. The amount of unbalance between the signals from channels is different in players. The balance control device for the stereo system is therefore switched from one level to the corresponding level for the corresponding player. FIG. 2 shows such a conventional balance control device. By switching between first and second switches 21, 22 which operate in combination with each other, the output of a compact disk player 23 or a video disk player 24 is selectively supplied to a balance control circuit 25. The selected two signals are so controlled as to have the same level by the balance control circuit 25, so that the left and right stereo signals having the same level are obtained from a left output terminal 26 and a right output terminal 27.
In some cases, however, for example, when the video disk player 24 is changed over to the compact disk player 23 in the circuit shown in FIG. 2, the stereo system may be held in an ill balanced state for some time until the control by the balance control circuit 25 is started. The balance control circuit 25 shown in FIG. 2 is capable of the balance controlling operation only when a monophonic signal is applied thereto (in the case the right and left stereo signals input are equal to each other). The balance controlling operation is carried out not constantly but only intermittently in some sources. For example, in a video disk player for a motion picture which contains many human conversations, the control is frequently carried out, but in a compact disk player mainly for music, balance control is scarcely carried out. Therefore, if the switching operation between sources is carried out in the above-described way, the balance control circuit 25 sometimes may control the signal from the compact disk player 23 in accordance with the value used for controlling the video disk player 24.